This invention relates to electro-optical removal of a plastic layer bonded to a metal tube, and more particularly to laser removal of the plastic layer without damage to any corrosion resistant layer bonded to the metal tube.
In the automotive industry, it is typical to create what are called "ISO" or "SAE" flared endforms on high pressure fluid conduits, particularly brake tubes. Automotive manufacturers mandate that the front and back flare faces be free from substantial polymeric layers. The manufacturers are concerned over a potential loss of assembly torque over the long term of a vehicle's life which could occur if the relatively soft polymeric material, eg. nylon, should extrude out of the sealing area and fitting compression area after assembly.
The polymeric material is present on the outer surfaces of the fluid conduits in order to greatly enhance the corrosion resistance of the metal tubing comprising the conduits. Thus, manufacturers of these conduits, especially when end use will be under a vehicle body, do not want to remove any more of this corrosion resistance-enhancing polymeric material than is necessary, eg. for example, not past a flare into the straight section of the tube. Further, the metal tubing generally has a corrosion resistant layer bonded to the metal tube outer surface. As such, it is highly desirable that any removal process not damage the corrosion resistant layer beneath the polymer, nor the outer surface of a bare metal tube (if no corrosion resistant layer has been bonded thereto).
Several methods have been tried, with varying levels of success. However, each method has serious drawbacks, preventing the use thereof. A rotary lathe cut method uses a chuck holder with lathe style square tool bits. It is mounted on a standard rotary head deburring unit. The method is simple and reliable; however, results revealed that the nylon does not easily machine off. Strings/burrs are left on, particularly at the transition line. Further, infinite adjustments and measurements would have to be made due to the tube O.D. variance, to attempt to prevent cutting through the substrate.
A method using rotary brushes employs the use of a grinding wheel head driving a brush. The tube was held and rotated by hand. The area where the coating is to be removed is forced into the brush, and the transition line is determined by locating a protective sleeve over the tube at the desired location. The sleeve used is about 2" long and held in place with a set screw. Unfortunately, this method requires a complex adjustment mechanism to compensate for brush wear. Further, it is very difficult to determine if only the nylon would be removed. Other brushes have been tried with no real success.
A square head punch method uses a blunt nose punch that has a fixed diameter hole that goes over the steel tube and pushes the nylon to a desired distance. The "pushed back" nylon material is then cut off and removed via a rotary lathe cut system. This method has the drawback that it would be impossible to predict the amount (thickness) of nylon removed or conversely, left on, and it would dig into any corrosion resistant coating.
A water jet knife method involves the use of a high pressure water jet system to cut and blow off a nylon coating without affecting a ZnAl substrate. The method involves rotating the tube at high rpm while a stationary high pressure water jet removes the coating. The travel speed of the jet was 12"/minute (=0.2"/sec.). The entire system uses 50 Hp of power (=37.3 KW). This method does appear to remove the nylon coating without affecting the corrosion resistant coating, it is forgiving to the O.D. and ovality variances and has a very fast cycle time. However, it is difficult and costly to have high rpm tube rotation; the system uses ultra clean water as the removal medium (which is expensive), and it is not economical to recycle the water. The water will be a problem to the exposed ends of the tube. A further drawback is that there are high maintenance costs for the system.
In German Patent No. DE 44 13 218, there is disclosed a device for the contactless removal of a layer of lacquer or plastic, for example PTFE, from a component by means of a pulsed laser beam. This method removes a coating without damage to a primer underneath. However, it appears that the laser energy is guided to any location within a semicircular work space. As such, it appears that the laser energy would not be rotatable around a workpiece, thus requiring rotation of the workpiece, which adds to production costs. Further, it does not appear that the disclosed method removes more than a single layer. A further drawback is that the disclosed machine, comprising articulated opto-mechanical arms, is large, complex and very expensive.
Thus, it is an object of the present invention to provide a method for the fast, efficient, precise and cost effective removal of one or multiple polymeric layers from a metal substrate. Further, it is an object of the present invention to provide such a method which will not damage the substrate or any corrosion resistant coating thereon. Still further, it is an object of the present invention to provide such a method which leaves a smooth transition portion to aid in subsequent connections. It is yet another object of the present invention to provide such a method which optionally leaves a residue for enhanced corrosion resistance on the removed surface. It is still another object of the present invention to provide such a method which is forgiving to outer diameter and ovality variances.